This page shows you how to display any 640x480 VGA source on your RGB monitor, utilizing an Extron Emotia (exact compatible models listed below).  There are other devices that are similar (I'll talk about those later), but the Emotia is so easy to use that I wanted to talk about it specifically.  Here's a quick rundown of how to use it:

First, plug your VGA source into the Emotia and use BNC cables to go from the Emotia to your RGB monitor.  Please note that it's a male VGA port, so you'll need to get the proper cable or gender changer, based on your VGA source.  All the cables you need should be available via the links on your right, but please double check your VGA source before buying, just to be safe: 

Here's how mine looks all plugged in.  Notice there is a "monitor output" connection on the far left, so you can connect a VGA monitor as well.  This is good if you need to experiment with different resolutions that your RGB monitor may not support.  Also, there's an S-Video and Composite out, but you probably won't need those.  Lastly, note the one black button between the VGA ports:

Then, turn on your VGA device and set it to 640x480.  Some RGB monitors accept higher resolutions, but just start with that for now.

If your picture looks weird, try pressing the black button in the rear of the unit between the two VGA connectors (that I mentioned above).  That should be all: No crazy configuration, no fumbling between the RGB monitor and a VGA monitor, it just works!:

A note about 240p:
When a 640x480 VGA device is passed through the Super Emotia, the signal is being converted to interlaced, making it 480i RGB.  If you set the front switch to "Non-interlace", it will convert the signal from 480i to 240p (320x240 progressive scan).  Most of the time, this may actually look much better.  This is especially true if you're making a Mame arcade machine, or playing emulators for older systems who's native resolution is 240p.  For more information, please check out my 240p page!

Compatible Emotia's:
The only versions of the Emotia to feature 240p output are the Extron Emotia, the Emotia Plus, the Super Emotia and the Super Emotia GX. Other models like the Super Emotia II or Emotia Xtreme have dropped their 240p capability.  

Other options:
At the moment, the only other "Downscaler" available is the MIMO Genius II.  It can be purchased directly from the manufacturer and requires a custom power supply (you can just use a 12v, 1A supply and use screw connectors).  Here's is Fudoh's full review with all the info you'd need: 
If you'd like more detailed information on how this works, or you're just curious about what other options there are, I highly recommend you check out Fudoh's page on these devices.  He's the same guy that has the excellent website about upscalers that I talk about in the RGB Guide

Extron RGB Interfaces:
You can use an Extron Rxi to help fine-tune the image, as well as add a few other options.  Fudoh expains in detail why you'd benefit from this in the link above.  Also, please check out the Extron Rxi page for more info.

Feel free to go back to the main VGA to RGB Monitor page, or even check out the page that talks about getting a special video card that outputs RGB.  If you're done, I highly recommend you check out the other great guides listed on the main page...especially the RGB Guide!